1. Field of the Invention--The invention generally relates to measurement and testing. More specifically, the invention relates to bore hole and drilling study, especially to formation logging by thermal study. The invention also generally relates to boring and penetrating the earth, and particularly to a bit and bit element structure. Another general aspect of the invention relates to electrical communications with acoustic wave systems and devices, especially those for well bore telemetering.
2. Description of the Prior Art--In many types of earth drilling, it is desired to know the temperature at the bottom of the well bore. Such measurement taking is difficult while drilling is in prograss, because the drill bit and drill pipe are occupying the well bore. Measurements while drilling (MWD) have long been a goal, especially in the oil industry. MWD refers to the taking of measurements while the drill pipe is in the ground, at or near bottom hole, with the drill bit either turning or stationary. Without this capability, taking measurements at the bottom of a hole being drilled is complicated, time-consuming, and expensive. The typical method of attempting to measure formation temperature is to stop drilling, pull the entire drill string from the hole, lower a wire-line instrument into the bore hole, and take the required measurements. Subsequently, the measuring device is removed from the hole, and the drill string is dropped back in place.
Other methods and equipment are known that attempt to take the measurement while drilling continues or at least without requiring removal of the drill stem. For example, U.S. Pat. No. 3,455,158 to Richter, Jr. et al, employs a transducer or thermocouple located in the drill bit. Due to the location of the thermocouple, the resulting measurement would be indicative of the temperature of the bit, the drilling fluid, and the formation. However, a formation temperature would not be readily evident, since the bit warms by friction and the circulating drilling fluid carries heat through the well bore. Another system is taught in U.S. Pat. No. 4,561,300 to O'Brien, which discloses a capsule-like temperature recorder that contains alloys of various melting temperatures. The capsules circulate with drilling fluid and are recovered for inspection at the wellhead. This system will show the highest temperature encountered by a capsule, but it would be unknown where in the well bore the capsule encountered the temperature. In addition, the temperature would likely be that of the surrounding drilling fluid and not the true formation temperature. A further system is taught in U.S. Pat. No. 3,701,388 to Warren, which discloses a method of drilling in which mud temperature is continuously measured at the wellhead by an in-line sensor. This temperature would be related only loosely to the formation temperature deep in the well bore.
The state of the art also is shown by patents relating to measurements taken after drilling is complete. For example, U.S. Pat. No. 3,363,457 to Ruehle discloses an infra-red detector that is lowered into the borehole after the drill string is removed. Thus, the detector takes the measurements after drilling is complete and after the well bore has had a considerable opportunity to stabilize its local formation temperatures. Further, U.S. Pat. No. 2,843,459 to Meiklejohn discloses a different type of tool that is used after drilling is complete. This tool ejects a chemical against the wall of a well bore, and the tool then employs temperature sensors to detect any resulting temperature increase within the well bore. The purpose and function of this tool is limited to detecting the heat of any resulting chemical reaction with the material of the bore wall and not to measure true formation temperature.
The state of the art is further demonstrated by a paper by Vagelatos, Steinman, and John, "True Formation Temperature Sonde (TFTS)," presented at the SPWLA Twentieth Annual Logging Symposium, June 3-6, 1979. This paper describes the importance of knowing the true formation temperature and describes a wire-line instrument that purports to measure this temperature by neutron gauging, wherein fast neutrons are directed into the formation and returning slow neutrons are detected. The method purports to penetrate the formation wall to a depth of 6 to 12 inches. This instrument would be used after the drill bit has been removed from the well bore. Although the method does not purport to enable measurement while drilling (MWD), it demonstrates the direction of the art.
Various patents have proposed telemetry systems for transmitting data from the well bore to the surface and are incorporated by reference for this teaching. U.S. Pat. No. 4,578,675 to MacLeod teaches a telemetry system that employs electrical current transmissions through the drill string to characterize formations. U.S. Pat. No. 4,393,485 to Redden discloses a post-completion monitoring system for tracking production information in a well. Temperature and pressure sensors are placed downhole in a producing well and are monitored by computer. Another system is disclosed in U.S. Pat. No. 4,520,468 to Scherbatskoy, in which a pulsar transmits data through the drilling fluid via pressure pulses.
Well temperature measurements are desired as a means of determining formation properties. Obtaining an accurate measurement of a downhole formation temperature is difficult not only because of physical difficulty and expense, but also because of technical problems, particularly when the desired measurement is the true formation temperature at any depth. The expression, "true formation temperature," refers to the rock temperature that existed at any depth before drilling began. One of the technical problems is that the act of drilling, itself, changes rock temperature in the vacinity of the well. The friction and cutting action of the drill bit adds energy to the rock, thereby increasing its temperature. Thus, at the moment when a formation is exposed for possible temperature measurement, the exposed area has a substantially altered temperature.
A further problem is that the action of the drilling mud significantly reduces the rock temperature in most situations. Drilling mud is injected in the well through the drill pipe, and it returns to the surface through the annular space between the outside of the drill pipe and the wall of the hole being drilled. The drilling mud may have several functions including lubrication, lifting the drilling debris out of the well, and cooling the system. Even if cooling is not the goal, it will occur. The rock wall itself will be cooled, and it will approach the temperature of the drilling mud. Because of the cool wall, the interior of the rock also will be cooled, as heat is conducted from the rock formation toward the cooled wall.
This cooling effect will extend into the rock for a certain distance from the face of the well bore. However, there will be some distance from the face of the well bore at which the temperature of the rock will remain at the true reservoir temperature. Several factors influence the size of this distance, including: the length of time the well bore surface has been exposed to the drilling mud; the heat transfer coefficient between the well bore and the mud, which depends on such factors as the degree of turbulence in the flowing mud and the properties of the mud; the physical properties of the rocks, such as density, heat capacity and thermal conductivity; and the characteristics and uniformity of the rock matrix as is related to such factors as its porosity and permeability and whether the pores are filled with water, oil or other material. Thus, the temperature actually measured by conventional equipment is altered from the true temperature of the earth as it existed before the drill bit penetrated the formation.
The problem of measuring true reservoir temperature is that, if a temperature sensor is placed within a well bore, either while drilling is in progress or while it is stopped, the sensor will measure the temperature of the mud and not that of the rock formation. Even if the sensor is placed against the surface of the rock wall of the well bore, it will measure the wall temperature and not the true formation temperature.
The art has not developed an apparatus or method that enables the true reservoir temperature to be determined, especially when drilling is in progress or is momentarily stopped. As drilling time is expensive, it would be important and significant to be able to determine true formation temperature accurately, rapidly, and without substantial interruption of drilling operations. Since rotation of the drill string must be stopped periodically to add additional drill pipe, it would be desirable to have a device that could take an accurate temperature measurement at the bottom of the well bore in no more than the time necessary to add such pipe. Of course, it would be equally as beneficial to take the measurement while the drill string is rotating. Either of such techniques could be termed "measurement while drilling."
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the apparatus and method of this invention may comprise the following.